The chemistry of azlactones has received considerable attention in the literature, mainly as it relates to amino acid and peptide chemistry. Azlactones are known to react with a variety of nucleophiles (e.g., alcohols, amines, mercaptans) by a ring-opening reaction to produce compounds containing ester, amide, or thioloester linkages. By contrast, bis(azlactones) are less well known, particularly those of the 2,2'-bis(2-oxazolin-5-one) class.
C. S. Cleaver and B. C. Pratt, J. Amer. Chem. Soc., 77, 1541, 1544 (1955), conducted the first systematic studies on the synthesis and characterization of bis(azlactones) having aromatic and hydrocarbon linkages to the 2-positions of the two azlactones. These compounds were used to prepare a new series of polyamides containing a regular arrangement of "head-to-tail" and "tail-to-tail" amide groups. Other investigators, e.g., M. Ueda, et al., J. Polym. Sci.:Polym. Chem. Ed., 16, 155 (1978), have expanded upon this method of polyamide synthesis. Diefenback, et al., Ger Offen. No. 2,516,978, Oct. 28, 1976, (Chem. Abstr., 86, 3112t, 1977), describe the use of compounds containing two azlactone rings linked by a tetramethylene group as a crosslinking agent for high solids and powder coatings. Humbert, et al., U.S. Pat. No. 4,092,298, teach the use of bis(azlactones), wherein the linking group is a hydrocarbon or halo-substituted hydrocarbon group, as novel crosslinking agents for hydroxy-containing polymeric powder coating compositions. In addition, Inata, et al., U.S. Pat. No. 4,291,152, teach the use of bis(azlactones), wherein the linking group between the azlactone rings may be ##STR1## where X is O, S, CO, or SO.sub.2, for the purpose of chain-extending linear, hydroxy-terminated polyesters.
.beta.-Hetero(nitrogen or sulfur)-substituted ethylene groups are not known in the art as linking groups between the 2-positions of azlactones, and the use of bis- and poly(azlactone) compounds, with these linking groups, suitable for the preparation of thermoplastic and thermosetting resins has not been disclosed.
Further, the preparation of bis(azlactones) in the prior art is generally accomplished via the Schotten-Baumann reaction of a dicarboxylic acid dichloride with an .alpha.-amino acid, followed by a dehydrative ring-closure reaction. Overall yields from this reaction sequence are generally poor, and the latter step is fraught with difficulties (see previously cited Cleaver and Pratt article). In addition, all known examples of bis(azlactones) are crystalline solids, often exhibiting high melting points and corresponding limited solubilities in the types of materials useful for the preparation of curable resins.